Tests for malabsorption
There are several tests that can be performed to help diagnose malabsorption:
Blood tests – These can be used to identify suspected malabsorption and are usually the first tests done. They are not specific because low levels of certain substances could be due to disorders other than malabsorption e.g. an unusual diet.
Blood carotene levels are useful to screen for malabsorption. Low levels of carotene in the blood suggest deficient absorption of fat-soluble vitamins or dietary deficiency. Serum carotene levels are generally low in people with fat malabsorption. Vitamin B12 and folate levels may also be used to screen for malabsorption. There are numerous causes of folate and B12 deficiency, and since the deficiencies often occur together and cause similar types of anemia, both must be measured to ensure proper diagnosis. Low calcium levels may result from either malabsorption of vitamin D or to binding of calcium to unabsorbed fatty acids. Vitamin K deficiency resulting from malabsorption may cause bleeding disorders. Anemia due to iron deficiency may be caused by malabsorption of iron in the first part of the small bowel. Normal levels of carotene, vitamin B12, folate, iron, calcium, phosphorus, albumin, and protein suggest that malabsorption is not a significant problem.
Another type of blood sampling can be used to test absorption. A substance can be administered orally and its concentration is then measured in the blood to provide a measure of absorptive capacity. The most commonly used test is the D-xylose test. A sugar called xylose is given orally and then measured in the blood 2 hours later.
Stool tests – Tests of the fat content of stool may be used to determine if fat malabsorption is present. Stool is collected over a period of 72 hours with the person consuming a diet containing 100g of fat per day. If the amount of fat in the stool is high, it suggests that the body is not absorbing fat.
Breath tests – Breath tests are another method of detecting malabsorption. They are most often performed to test for lactose intolerance. Lactose is given by mouth and the subject’s breath is analyzed for the presence of hydrogen gas. If lactose is being malabsorbed, colonic bacteria will work on the lactose to produce hydrogen gas which will be exhaled by the patient and measured in his or her breath.
Other tests – Tests such as biopsies of the small intestine (usually performed using an endoscope passed through the mouth into the intestine) are used to diagnose certain malabsorptive conditions. Special tests to image organs such as the pancreas are also useful in some cases.